The thin film transistor has always been a switch control element of panel display or an integrated element of peripheral driving circuit. In addition, the thin film transistor is also widely studied and used in some fields such as sensor, storage, processor, etc. Currently, the thin film transistor which is widely used by the industry is mainly the traditional silicon-based thin film transistor, such as amorphous silicon thin film transistor and polycrystalline silicon thin film transistor. However, with the continuous development of the display technique, these silicon-based thin film transistors are beginning to fail to meet the higher and higher demand of people to flat panel display technique. In the amorphous silicon thin film transistor, the main disadvantages are low mobility, easily degraded performance, and the like, which greatly restrict its applications in the following aspects such as OLED pixel driver, the integration of LCD and OLED peripheral driving circuit, etc. Moreover, the high process temperature, high fabrication cost and poor uniformity make the polycrystalline silicon thin film transistor unsuitable for large size panel display application. Therefore, in order to develop the panel display technique, the metal oxide thin film transistor as a novel thin film transistor technique is recently widely studied.
The metal oxide thin film transistor has low process temperature, low process cost, high carrier mobility as well as uniform and stable device performance, which not only possesses the advantages of both amorphous silicon thin film transistor and polycrystalline silicon thin film transistor, but also possesses advantages such as high visible light transmittance, etc., and it is very promising for application in the next generation of large size, high resolution, and high frame frequency transparent display. The channel layer material used in the metal oxide thin film transistor mainly comprises zinc oxide (ZnO), indium oxide (In2O3), indium gallium zinc oxide (GIZO), zinc tin oxide (ZTO), indium zinc oxide (IZO), indium zinc tin oxide (TIZO), tin oxide (SnO2), stannous oxide (SnO), cuprous oxide (Cu2O), etc.
In the process for fabricating the metal oxide thin film transistor, in order to obtain proper threshold voltage, the channel layer needs a high resistance layer with a low carrier concentration, while in order to reduce parasitic resistance, the source and drain need a low resistance layer with a high carrier concentration. Unlike the polycrystalline silicon thin film transistor, the resistances of the source and drain of the metal oxide transistor cannot be reduced by the manner such as ion implantation, etc. Therefore, an additional metal layer with low resistance is needed for the source and drain, which increases the complexity of the fabrication process.